The present invention relates to a circuit protector of the type wherein upon flowing of excess current across a pair of terminals, a heat-sensitive plate bends to snap its movable contact disengaged from a fixed contact to thereby cut off the excess current, the heat-sensitive plate for use in the circuit protector and a method of making the heat-sensitive plate. More particularly, the invention pertains to a heat-sensitive plate suitable for miniaturization of the circuit protector.
FIGS. 1A and 1B depict an example of a conventional circuit protector disclosed in Japanese Patent Application Laid-Open Gazette No. 7-27040. The circuit protector of this example has substantially a block-shaped base 12 molded of synthetic resin, and a box-shaped cover 13 which is also a molding of synthetic resin and whose bottom opening is blocked by the base 12. When the cover 13 is mounted on the base 12, lugs 16 formed integrally with the base 12 on one side thereof engage holes 17 made in one side wall of the cover 13 to prevent it from becoming dislodged. On the base 12 there are planted terminal plates 14 and 15 vertically extending therethrough. The terminal plates 14 and 15 are mounted in parallel with each other on the base 12 and spaced apart but side by side in the lengthwise direction thereof.
On one side of the first terminal plate 14 that lies in the cover 13 shown in FIG. 1B, there is mounted a fixed contact 18. The second terminal plate 15 carries a heat-sensitive plate 19 mounted on one side thereof. On the top of the base 12 there is planted near the first terminal plate 14 at the side opposite from the second terminal plate 15 a pin 32 to which a coil spring 28 is fixed at one end. The coil spring 28 has its upper end portion received in a hole 31 made in the lower end face of a reset rod 27, and pushes it up. The reset rod 27 has a thin rectangular insulating plate 33 that extends from its one side surface and across substantially the entire surface area of the second terminal plate 15 on which the fixed contact 18 is mounted.
Even when the reset rod 27 is at its lowermost position, its upper end portion projects upwardly from a guide hole 29 made in the top of the cover 13. The heat-sensitive plate 19 extends across the first terminal plate 14 with its tip or forward end portion projected beyond the far side of the plate 14 to keep a movable contact 24 in touch with the fixed contact 18 at all times.
The heat-sensitive plate 19 is a virtually square temperature-sensitive bimetal sheet, which has a U-shaped slit 22 extending along the inside of its marginal edge to form a tongue-shaped movable piece 23. The movable contact 24 is placed near the free end of the movable piece 23. One side of a frame portion 19F just opposite the free end of the movable piece 23 is press-bent centrally thereof to form a V-shaped bend 25 so that this one side is bent into a shallow V-letter shape as a whole. By this, tensile stress is applied to those regions of the frame portion 19F immediately adjoining the V-shaped bend 25 at both sides thereof, and the angle between the surfaces of the both frame regions adjacent the both edges of the V-shaped 25 is made larger than 180 degrees on the side of a large expansion coefficient side of the bimetal sheet. As a result, the heat-sensitive plate 19 is so bent as to swell toward the movable contact point 24 at room temperature, thereby biasing the free end of the movable piece 23 toward the fixed contact 18. The heat-sensitive plate 19 is fixedly secured to the second terminal plate 15 by means of rivets 21 which are inserted through holes 21H made in the frame portion 19F.
When the heat-sensitive plate 19 is not mounted on the second terminal plate 15, the movable piece 23 is obliquely bent toward the fixed contact 18 side. Accordingly, when the heat-sensitive plate 19 is mounted on the second terminal plate 15, the movable contact 24 is resiliently pressed against the fixed contact 18 by the spring force of the movable piece 23 and the heat-sensitive plate in combination as depicted in FIG. 2B, establishing electric connections between the first and second terminal plates 14 and 15. This is a normal state, wherein the insulating piece 33 is urged upward by the coil spring 28 with its upper edge held in abutting relation to the lower marginal edge of the movable contact 24.
The bimetallic heat-sensitive plate 19 has a higher thermal expansion coefficient on the side facing the fixed contact 18 than on the opposite side. When the heat-sensitive plate 19 generates heat due to excess current flow between the fixed and movable contacts 14 and 24 and its temperature rises accordingly, the heat-sensitive plate 19 tends to bend in a direction in which it is concavely curved on that side facing the fixed contact 18. Hence, when overcoming the deformation stress initially applied thereto, the heat-sensitive plate 19 snaps into a reverse curvature, disengaging the free end of the movable piece 23 from the fixed contact 18 and hence cutting off the current flow between the first and second terminal plates 14 and 15. As a result, the insulating piece 33 disengages from the movable contact 24, and is moved up by the coil spring 28 as shown in FIG. 3A and pushed into between the movable contact 24 and the fixed contact 18 as depicted in FIG. 3B. Then the insulating piece 33, which has a rib 34 extending from its base along one side of the reset rod 27, is positioned with the upper end of the rib 34 abutting against the inner surface of the top of the cover 13. Accordingly, even if the temperature of the heat-sensitive plate 19 drops down to about room temperature after cutting off the current flow and the heat-sensitive plate 19 and the movable piece 23 tend to return to their initial state, the movable contact 24 remains abutting against the insulating piece 33, inhibiting the current flow.
The circuit protector in this state is reset by pressing down the reset rod 27 against the coil spring 28 to push down the insulating piece 33 from between the fixed contact 18 and the movable contact 24, bringing the latter into engagement with the former. By releasing the reset rod 27 in this state, the insulating piece 33 is brought up by the coil spring 28 until its upper edge abut against the lower marginal edge of the movable contact 24, thereafter being held at this position.
The conventional protector described above has the U-shaped slit 22 formed inside the marginal edge of the heat-sensitive plate 19 so as to form the movable piece 23. In order that the heat-sensitive plate 19 may snap into the opposite direction of curvature, depending on which of the thermal expansion stress by the bimetal sheet and the tensile stress applied to the frame portion 19F by the V-shaped bend 25 is larger, it is necessary that the frame portion 19 surrounding the movable piece 23 be relatively wide. This constitutes an obstacle to miniaturization of the heat-sensitive plate 19.
In addition, the direction of extension of the movable piece 23 is at right angles (i.e. horizontal) to the direction of movement of the reset rod 27 (the vertical direction in FIGS. 1A, 2A and 3A). Furthermore, the free end portion of the heat-sensitive plate 19 which supports the movable piece 23 extends from the movable contact 24 in a direction opposite to the fixed end of the heat-sensitive plate 19. To enhance the reliability of the operation of the heat-sensitive plate 19, its free end portion needs to be long. However, this gives rise to a problem that the circuit protector is long in the direction of extension of the heat-sensitive plate 19 (that is, the longer side of the rectangular top of the cover 13) is long. Therefore, it has been impossible to meet a demand for circuit protectors of miniature size which has grew strong with the recent miniaturization of electronic equipment.
FIGS. 4A and 4B show another example of the heat-sensitive plate 19 for use in the conventional circuit protector depicted in FIGS. 1A, 1B, 2A, 2B, 3A and 3B. The illustrated heat-sensitive plate 19 has a protrusion 19a protrusively provided on a low expansion coefficient side 19L of a substantially rectangular bimetal sheet by press working of its central portion. The protrusion 19a is a little smaller in diameter than the shorter side of the bimetal sheet as shown in FIG. 4A and spherical in cross-section as shown in FIG. 4B. By forming such a protrusion 19a, stress is applied to the surrounding region 19d to slightly bend it into a shallow funnel shape in the same direction as that of the protrusion 19a. On the high expansion coefficient side 19H of the bimetallic element, there is mounted the movable contact 24 adjacent one of its shorter sides. As the temperature of the heat-sensitive plate 19 increases, the high expansion coefficient side 19H expands and is urged to become convex, applying stress to the surrounding region 19d. The instant when this stress overcomes the deformation stress by the formation of the protrusion 19a, the surrounding region 19d snaps its direction of curvature reversed (but the direction of curvature of the protrusion 19a remains unchanged).
To make the heat-sensitive plate 19 of the above construction snap into the opposite direction of curvature, it is necessary to form such a relatively large-diametered protrusion 19a as depicted in FIG. 4A. In addition, this conventional heat-sensitive plate 19 has the defect of a wide range of variations in the temperature characteristic of the reversal action.